using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace WXB_timus
{
    class Program
    {
        static void Main(string[] args)
        {
            GetAbsoluteSum();
        }

        //timus 1068
        static void GetAbsoluteSum()
        {
            int InputNum = Convert.ToInt32(Console.ReadLine());
            if (Math.Abs(InputNum) > 10000)
            {
                return;
            }
            int NumSum = 0;
            if (InputNum < 0)
            {
                for (int i = InputNum; i <= 1; i++)
                {
                    NumSum += i;
                }
            }
            else if (InputNum > 0)
            {
                for (int i = 1; i <= InputNum; i++)
                {
                    NumSum += i;
                }
            }
            else
            {
                NumSum = 1;
            }


            Console.WriteLine(NumSum);
        }

        //timus 1025

    }


}

